setup illustrated in Fig.7.20. Here a short length of PCF is sandwiched between
input and output SMFs (such as the conventional industry standard G.652fibers) to
form an inline core-cladding intermodal interferometer. Different higher-order
cladding modes can be excited in the PCF when the fundamental mode (the LP 01
mode) in the single-modefiber encounters abruptfiber tapers formed at the inter-
faces between the PCF segment and the single-mode input and outputfibers.
When light from a broadband source is sent through the device, the interference
between the core and cladding modes in the PCF sensing section will result in a
transmission-versus-wavelength interference pattern [ 3 ]. Since the higher-order
mode is a cladding mode of a glass-air waveguide, any change of the outer
refractive index caused by immersion of the PCF sensing segment in a liquid will
change the propagation constant of the higher-order mode and consequently will
cause a shift in the interference fringes.
7.6.2 Liquid Infiltration Sensor
The infiltration of afluid sample into the PCF can be achieved by simply immersing
thefiber end in the liquid. The liquid then enters thefiber holes through capillary
action. However, afterwards if the liquid needs to be removed from the PCF, which
could be required in multi-step biochemical processes, then either a vacuum or a
pressure needs to be applied to thefiber end.
Because the air holes in a PCF run along the entire length of thefiber, this
feature is being used to enable long interaction lengths between the excitation light
and small volumes of an analyte. One such embodiment involves fabricating a PCF
that has a side-channel opening running along thefiber, which can be used for
containing a liquid analyte that can be analyzed by means of surface enhanced
Raman scattering (SERS) sensing [ 47 – 49 ].
LP 01 core mode is
distributed among
the cladding modesCladdingSMF 1Cladding modes are
recombined into a
modified LP 01 core modePhotonic crystal fiber SMF 2The analyte
interacts with the
cladding modesLP 01 core mode 1 LP 01 core mode 2LP 01 core mode is
distributed among
the cladding modesCladdingSMF 1Cladding modes are
recombined into a
modified LP 01 core modePhotonic crystal fiber SMF 2The analyte
interacts with the
cladding modesLP 01 core mode 1 LP 01 core mode 2Fig. 7.24 Concept of an inline core-cladding intermodal interferometer based on the mode-field
mismatch method
222 7 Optical Probes and Biosensors